Method of controlling an ejector of an injection molding machine

ABSTRACT

An ejector mechanism is driven by a servomotor. An ejector pin in the ejector mechanism is made to perform a motion such that the ejector pin reaches a predetermined protrusion limit position beyond a position where the removal of a molded product from a cavity or core is completed after making a check for positioning, and a plurality of cycles of reciprocating motion of a short amplitude such that the ejector pin neither retracts beyond a position where the removal of the molded product from the cavity or core is started nor protrudes to the protrusion limit position, without requiring a check for positioning.

TECHNICAL FIELD

The present invention relates to an improvement of a method in which amolded product, which remains in a cavity or a core of a mold afterbeing injection-molded by an injection molding machine, is released fromthe cavity or the core to be dropped outside by protruding an ejectorpin to the cavity or the core.

BACKGROUND ART

In a known ejector apparatus, a molded product, which is difficult to bereleased from a mold, is forcedly released or unmolded from a movablemold by repeatedly executing the protruding motion of an ejector pin. Inthe conventional ejector apparatus, however, a reciprocating motion inwhich the ejector pin is protruded from an unmolding start position to agiven protrusion completion position and is returned again to the unmoldstart position is simply repeated. Therefore, the ejector apparatus ofthis type has a disadvantage such that if the number of repetitions ofprotruding motion is increased to perform a reliable unmoldingoperation, the cycle time of the molding operation increases.

Especially when the molded product sticks to the ejector pin itself, itis necessary to repeatedly execute such a protruding motion, resultingin a marked increase in cycle time. In many of the conventional ejectorapparatuses, the return position of an ejector rod is set at a positionfurther behind the end face of an ejector plate from which the ejectorpin is erected, so that in many instances a delay in motion is oftencaused by a useless increase in motion stroke of the ejector rod. Whenthe molded product, sticking strongly to the ejector pin, is not removedfrom the mold by single protruding motion, the ejector pin is stronglypushed out at every protruding motion to give an impact force to themolded product. This gives rise to a problem such that a tear isproduced on the molded product or the ejector pin runs through themolded product, producing deformation.

Unexamined Japanese Patent Publication No. 59-42941 discloses an ejectorapparatus in which a plurality of cycles of protruding motion areexecuted in a short period of time by starting the next protrudingmotion of the ejector pin before completely returning the ejector pin tothe unmold start position after the protrusion of the ejector pin hasbeen completed. In the ejector apparatus of this type, only the ejectorpin protrusion start position in the second and subsequent cycles ofreciprocating motion can be changed, and the ejector pin protrusioncompletion position is always constant in any cycle of protrudingmotion. Therefore, this gives rise to a problem such that the unmoldingoperation according to the unmold property of molded product cannot beperformed. Also, a plurality of mechanical detection switches fordetecting the position of the ejector pin are arranged to check thereturn position and the protrusion completion position of the ejectorpin, so that the construction of the ejector apparatus becomes complex.Thus, the return position and the protrusion completion position of theejector pin cannot be changed (the installation positions of thedetection switches cannot be changed either) while the protruding motionare repeated continuously. That is to say, as described above, theejector apparatus invites a problem such that an unmolding operationaccording to the unmold property of the molded product cannot beperformed. Further, a reversal motion is carried out by detecting thereturn position and the protrusion completion position of the ejectorpin by means of the detection switches. Therefore, the motion is delayedby the time required for the detecting operation, causing an increase incycle time.

To reliably perform the unmolding operation of the molded product, anair ejector is sometimes used in combination with the ejector pin forthe unmolding operation. However, the air ejector is used only toprevent the unmolded piece from being hindered from dropping by ridingon the protruding portion of the mold or by sticking to the mold due tostatic electricity, unmolding agent, etc. Thus, the air ejector will notproduce any effect unless the molded piece is separated completely fromthe movable mold during the unmolding operation and is not capable ofremoving the molded piece from the movable mold as long as the moldedpiece remains sticking hard to the mold.

Unexamined Japanese Patent Publication No. 6-114897 and UnexaminedJapanese Patent Publication No. 6-170897 disclose a method foreffectively dropping the molded product from the mold by making theejector pin exert both protruding motion and fine vibration (or swingingmotion). However, for the minute vibration or the swinging motion,advancing is performed by a hydraulic cylinder and retreating isperformed by the restoring force of a return spring, so that thedistance of advance and the distance of retreat are equal to each other,and either of the retreat limit or the advance limit in minute vibrationis always the minute vibration start position (a dropping position wherethe molded product drops surely or a protruding start position). Withthis method, therefore, only an extremely limited kind of motion can begiven to the ejector pin. Therefore, according to these prior art, it isdifficult to produce an ejector motion of a kind most suitable to thedropping of the molded product.

DISCLOSURE OF THE INVENTION

An object of the present invention is to eliminate the abovedisadvantages of the prior art, and to provide an ejector control methodfor an injection molding machine, in which unmolding of a molded productcan reliably be performed without causing deformation of the moldedproduct even when the molded product sticks hard to a movable mold or anejector pin.

In order to achieve the above object of the present invention, anejector mechanism is driven by a servomotor, whereby an ejector pin inthe ejector mechanism is given a motion such that the ejector pinreaches a predetermined protrusion limit position beyond a positionwhere the removal of a molded product from a cavity or core is completedwhile performing positioning confirmation, and a plurality of cycles ofmotion of a short amplitude such that the ejector pin neither retractsbeyond a position where the removal of the molded product from thecavity or core is started nor protrudes to the protrusion limit positionwithout any positioning confirmation.

In the ejector control method according to a first mode of the presentinvention, the ejector pin is first protruded at a predetermined speedfrom a predetermined retracted position to the predetermined protrusionlimit position by one protruding operation, and the plural cycles ofreciprocating motion of the ejector are then started from the protrusionlimit position.

As a result, an unmolding operation can be performed by using the coreor cavity of a movable mold in place of a stripper plate, and a reliableunmolding operation can be performed even when the molded product sticksto the ejector pin. In this case, since the protrusion limit of theejector pin during vibration is the predetermined protrusion limitposition, there is no wasteful motion of ejector pin, and the unmoldingoperation can be performed in a short period of time. Also, since theunmolding operation is performed by the vibration of ejector pin, anylarge inertia will not act on the molded product unlike the case wherethe unmolding operation is performed at a stretch by making the ejectorpin perform a protruding motion of full stroke, and a problem such thatthe removed molded product is scattered inadvertently can be solved.

In the ejector control method according to a second mode of the presentinvention, the ejector pin is first protruded from a predeterminedretracted position to a position slightly beyond a position where theremoval of the molded product from the cavity or core is started by oneprotruding operation, and a plurality of cycles of reciprocating motionof the ejector are then performed from the protrusion position so thatthe protrusion distance is larger than the retraction distance, and theejector reaches the predetermined protrusion limit position by the finaladvance motion in the reciprocating motion.

As a result, even when the draft is insufficient, and the molded productsticks hard to the ejector pin at all stages from the start of moldrelease to the completion of mold release, the unmolding operation canbe performed without producing deformation such as tear and penetrationof ejector pin.

Also, according to the present invention, a servomotor is used as adriving source, and an ejector rod is vibrated in a manner such that theunmold start position is the ejector pin retraction limit, so that theejector rod does not move wastefully, and a high-speed vibration ofejector pin can be achieved. Further, by conducting a positioningconfirmation for the movement command to the servomotor unexecuted, adelay caused by the operation for checking the positioning iseliminated, so that the speed of the ejector pin vibrating motion isfurther increased. Thus, the cycle time can be shortened accordingly,and a reliable unmolding operation in which deformation of moldedproduct is prevented can be performed by more minute vibration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic view of an ejector mechanism and the surroundingsthereof in a motor-driven injection molding machine to which the methodof the present invention is applied;

FIG. 1B is a functional block diagram showing the outline of a principalpart of a control unit for controlling the motor-driven injectionmolding machine shown in FIG. 1A;

FIG. 2 is a flowchart showing the outline of control software forejector stored in the control unit in accordance with this embodiment;and

FIGS. 3A and 3B are timing charts showing examples of ejector pinmotion.

BEST MODE FOR CARRYING OUT THE INVENTION

In an injection molding machine, as shown in FIG. 1A, a moving platen 3moves towards or away from a stationary platen 4 under the control of aservomotor (not shown). A movable mold 8 and a fixed mold 9 are attachedto the moving platen 3 and the stationary platen 4, respectively. InFIG. 1A, reference numeral 7 denotes a product molded by injecting resinfrom an injection cylinder 5 into a cavity formed in the movable mold 8.

An ejector mechanism 1 on the injection molding machine comprises ballthreads 10, 10 attached rotatably to the back face (a face opposite tothe face on which the movable mold 8 is attached) of the moving platen 3of the injection molding machine, a pusher plate 12 provided, at thecenter thereof, with an ejector rod 2 passing through the moving platen3, guide rods 13, 13 for guiding the pusher plate 12 in such a manner asto move towards or away from the moving platen 3, a servomotor M forejector, which rotates the ball thread 10, 10, a timing belt 14, etc.

At both sides of the pusher plate 12, ball nuts 11, 11, which arescrewed onto the ball threads 10, 10, are integrally fixed. By drivingthe servomotor M to rotate the ball threads 10, 10, the pusher plate 12is moved along the guide rods 13, 13, causing the ejector rod 2 to beprotruded from the surface (the surface to which the movable mold 8 isattached) of the moving platen 3, or retracted therefrom.

The ejector rod 2 protruding from the surface of the moving platen 3further passes through a through hole formed in an attaching plate 15 ofthe movable mold 8, pushes an ejector plate 16 of the movable mold 8against the elastic force of return springs 17, and pushes moldedproducts 7 by using the tip ends of ejector pins 6 provided integrallyon the ejector plate 16, whereby the molded products 7 are removed fromthe core of the movable mold 8, in performing unmolding operation.

FIG. 1A shows a state in which the injection molding machine hascompleted injection (the mold is being clamped). The unmolding operationis not performed in this state, but is performed in a state in which themoving platen 3 moves away from the stationary platen 4, and the movablemold 8 and the fixed mold 9 are open.

As described above, the construction itself of the ejector mechanism 1is completely the same as that of the conventional motor-driveninjection molding machine.

As shown in FIG. 1B, a control unit 100 for controlling the injectionmolding machine, including a CPU for CNC 107, which is a microprocessorfor numerical control, a microprocessor for programmable machinecontroller (hereinafter referred to as a CPU for PMC) 111, and amicroprocessor for servo control (hereinafter referred to as a servoCPU) 103, and is designed to transmit information betweenmicroprocessors by selecting mutual input/output via a bus 110.

The CPU for PMC 111 stores a sequence program for controlling thesequence motion of the injection molding machine, and the like, and isconnected to a nonvolatile RAM 112 used for the temporary storage ofarithmetic data. Connected to the CPU for CNC 107 are ROM 104 storingcontrol software for controlling each axis of the injection moldingmachine and the like and a RAM 105 used for the temporary storage ofarithmetic data. The RAM 105, which is a nonvolatile reloadable memory,stores various molding conditions and setting values, parameters, macrovariables, etc. as well as a movement command program for controllingthe drive of the ejector mechanism 1 (hereinafter simply referred to asa movement command program).

Connected to the servo CPU 103 is a nonvolatile RAM 101 used for thepreservation of programs for servo control and the temporary storage ofdata, and servo amplifiers for driving a servomotor of each axis forejector, for mold closing, for screw rotation, for injection, etc. basedon the command from the CPU 103 (In FIG. 1B, only a servo amplifier 102for a servomotor M for ejector is shown, and other servo amplifiers areomitted). A signal from a pulse coder P provided on the servomotor M forejector is fed back to the servo CPU 103, whereby the current rotationalposition of the servomotor M for ejector, calculated on the basis of thefeedback pulse of the pulse coder P, that is, the current position ofthe ejector rod 2 is checked by the CPU for CNC 107.

An input-output circuit 109 is an input-output interface for receivingsignals from limit switches disposed at various positions of theinjection molding machine or a control panel, or for transmittingvarious commands to peripheral devices and the like of the injectionmolding machine.

A manual data input device 106 with display, being connected to the bus110 via a CRT display circuit 108, is used to perform the selection offunction menu, input operation of various data, and the like. It isprovided with ten keys for inputting numerical data and various functionkeys. The movement command program stored in the RAM 105 can berewritten for the convenience of user by the manual data input devicewith display 106.

By the above-described configuration, CPU for CNC 107 performs pulsedistribution for the servomotors of the axes based on the controlprogram of the ROM 104 and the data such as molding conditions of theRAM 105. On the other hand, the servo CPU 103 executes so-called digitalservo processing by executing the servo control such as position loopcontrol, speed loop control, current loop control, as in the case of theconventional injection molding machine, based on the pulse-distributedmovement command to each axis and the feedback signals for the positionand speed detected by a pulse coder of each axis.

FIG. 2 is a flowchart showing the outline of control software, for anejector, stored in the ROM 104 to execute the movement command program.This processing is sequentially executed by the CPU for CNC 107 onreceipt of the ejector operation command from the CPU for PMC 111 whichcarries out sequence control of the whole.

The CPU for CNC 107, which starts the execution of movement commandprogram based on the control software for ejector on receipt of theejector operation command from the CPU for PMC 111, first reads thefirst one block from the movement command program (Step S1), anddetermines whether or not this one block relates to the movement command(Step S2). If this one block relates to the movement command, pulsedistribution is executed based on the movement command of this oneblock, the movement distance, and preset movement speed, the servomotorM for ejector is driven through the servo CPU 103 and the servoamplifier 102, and the ejector pins 6 are moved by operating the ejectormechanism 1 (Step S3). After the distribution is completed (Step S4), itis determined whether or not positioning confirmation is necessary (StepS5). If check of positioning confirmation is commanded for that block,the mechanism waits until the positioning is completed, that is, untilthe difference between the position command and the actual position(positional difference) comes within a predetermined value range (StepS6). If a command requiring to ignore positioning is present, theprocessing moves to the Step S1 immediately after the completion ofdistribution.

Subsequently, the CPU for CNC 107 repeatedly reads the next one blockand repeatedly executes the same processing as described above. Finally,when the end command is read from the movement command program, allprocessing for unmolding operation is completed, and an ejectoroperation completion signal is outputted to the CPU for PMC 111.

Next, the operation of the ejector mechanism 1 in accordance with afirst embodiment will be described with reference to FIG. 1A.

In the unmolding operation according to this embodiment, the tip of theejector pin 6 is first protruded to a protrusion completion position E'beyond an unmolding completion position C'. Next, the retreat limit ofthe ejector pin 6 is set at an unmolding start position A' or a positionB' between the unmold start position A' and the unmold completionposition C', and a vibrating operation is performed with the protrusionlimit being a position D' between the unmold completion position C' andthe protrusion completion position E'.

This embodiment, which enables the removal of the molded product 7 fromthe movable mold 8, is effective when the molded product 7 sticks hardto the tip of the ejector pin 6. The tip positions A, B, C, D, and E ofthe ejector rod 2 corresponding to the tip positions A', B', C', D', andE' of the ejector pin 6, respectively, are shown in FIG. 1A. Theoperation times of the ejector rod 2 corresponding to the unmoldingoperation by the movement command program in this embodiment is shown inFIG. 3(a).

A tip position O shown in FIG. 1A is one example of a set originalposition for return of the ejector rod 2. The set original position forreturn of the ejector rod 2 can be set at any position on the retractionside from the position A where the tip of the ejector rod 2 abuts on theejector plate 16. Generally, after the movable mold 8 is installed byreturning the tip of the ejector rod 2 to the position O (on theretraction side from the surface of the moving platen 3) as shown inFIG. 1A, the ejector rod 2 is once protruded to detect the position Awhere the tip thereof abuts on the ejector plate 16, and then retractedslightly from this position to set the once-set original position O forreturn again, by which a loss of operation time caused by a wastefulmovement of the ejector rod 2 is prevented. At the same time, the returnof the ejector plate 16 to the original position is made by the returnsprings 17. Needless to say, if such setting is performed, the setoriginal position O for return will coincide substantially with theposition A.

When the movement command program is executed by the ejector controlsoftware, high-speed processing is performed omitting positioningconfirmation for the vibrating operation of the ejector pin 6 betweenthe position B' and the position D', and sure positioning is performedby checking the positioning for the first protruding operation of theejector pin 6 to the position E' beyond the unmold completion positionC' and for the returning operation to the set original position O forreturn of the ejector rod 2. The point where the check for positioningis executed is indicated with a small circle marks in FIG. 3A. FIG. 3Ashows the operation time of the ejector rod 2, and does not show theoperation time of the ejector pin 6 itself. However, the operation timeof the ejector pin 6 is completely the same as the operation time of theejector rod 2 in the interval between the position A and the position Ein FIG. 3A. The retraction limit of the ejector pin 6 during vibrationmay be the unmolding start position A'. But, in effect, when the ejectorpin 6 is retracted, the molded product 7 has to be caught by the cornerof the core or cavity (cavity in the example of FIG. 1A) of the movablemold 8 in place of a stripper plate. Thus, by setting the retractionlimit to the position B' on the protrusion side from the unmold startposition A', the vibration stroke of the ejector pin 6 can be shortened,so that unmolding operation can be performed at a higher speed. Sincethe vibration stroke of the ejector pin 6 is short, an inertia acting onthe molded product 7 when the molded product 7 is finally released fromthe ejector pin 6 can be reduced, whereby the problem such that themolded products 7 are scattered around to hinder the molded products 7from being collected efficiently is overcome.

Next, the operation of the ejector mechanism 1 in accordance with asecond embodiment will be described with reference to FIG. 1A.

First, the tip of the ejector pin 6 protrudes to the unmold startposition A' or the position B' which is slightly beyond that position.Then, the ejector pin 6 protrudes to the protrusion completion positionE' while vibrating in a manner such that the protrusion distance islarger than the retraction distance.

The operation time of the ejector rod 2 corresponding to the unmoldingoperation of the movement command program in this embodiment is shown inFIG. 3B. In FIG. 3B, R and G indicate the retraction distance andprotrusion distance of the ejector pin 6, respectively. The values ofthe protrusion distance to be increased by one vibration cycle (G-R),and the protrusion distance G and the retraction distance R in onevibration cycle are determined in advance appropriately according to thenumber of vibration cycles between the position B' and the unmoldcompletion position E' in FIG. 1A.

This operation is especially effective when the draft is insufficient,and the molded product sticks hard to the ejector pin at all stages fromthe start of mold release to the completion of mold release for themovable mold 8.

Thus, if the movement command program shown in FIG. 3(b) is executedbased on the ejector control software shown in FIG. 2, high-speedprocessing can be performed by omitting positioning confirmation for thevibrating operation of the ejector pin 6 to be carried out between theposition B' and the position E'. On the other hand, for the finalprotruding operation of the ejector pin 6 to the position E' and thereturning operation of the ejector rod 2 to the set original position Ofor return, indicated by a small circle mark in FIG. 3(b), reliablepositioning is performed by the check for positioning.

The operation time of the ejector pin 6 is completely the same as theoperation time of the ejector rod 2 in the interval between the positionB and the position E in FIG. 3B. In this case as well, an inertia actingon the molded product T when the molded product 7 is finally removedfrom the ejector pin 6 is small, so that the molded products 7 can beprevented from scattering around.

Two examples of operation of an ejector mechanism have been describedabove. In either case, the retraction limit of the ejector rod 2 is setso that the ejector rod 2 will not retract beyond the position A of theejector rod 2 corresponding to the unmolding start position A' of theejector pin 6 during the vibration the ejector pin 6, so that the tip ofthe ejector rod 2 will never be separated from the ejector plate 16.Therefore, useless independent movement of the ejector rod 2, which willnot accompany the operation of the ejector pin 6, is eliminated, wherebyhigh-speed processing resulting from the omission of positioningconfirmation, as well as high-speed vibration of the ejector pin 6, canbe achieved.

Further, if a movement command program for controlling the ejectormechanism 1 is prepared as occasion arises, various unmolding operationscan be performed according to the unmold characteristics of the moldedproduct 7 in addition to the above two examples. For example, after thetip of the ejector pin 6 is protruded to the unmold start position A',it may be protruded further to the unmold completion position C' whilegradually increasing the amount of protrusion, and finally the tip canbe protruded with one rush to the position E' beyond the unmoldcompletion position C'. Alternatively, in the example of FIG. 3A, anunmolding operation may be performed by gradually decreasing thevibration amplitude in the second vibration cycle and after.

This is because the servomotor M for ejector is controlled by the CPUfor CNC 107 based on the control software in the ROM 104 and themovement command program in the RAM 105. In a conventional injectionmolding machine, in which a plurality of mechanical detection switchesfor detecting positions are arranged on the ejector rod 2 to control theprotruding position of the ejector pin 6, needless to say, the controlof the operation as shown in FIG. 3(b), in which the protruding positionof the ejector pin 6 undergoes a change during a series of vibratingoperations, cannot be carried out.

As described already, in the configuration of this embodiment, themovement command program in the RAM 105 can be rewritten arbitrarily byoperating the manual data input device with display 106. Therefore, theuser can arbitrarily devise an ejector operation for unmolding operationdepending on the characteristics of molded product, and can make theejector device perform the ejector operation.

The user can give the control unit 100 an automatic programming functionfor automatically preparing the movement command program as describedabove by selecting an operation pattern as shown in FIG. 3A or FIG. 3Band by inputting the data such as the set original position O for returnof the ejector rod 2, the positions A, B, C, D, and E, the amplitude(D-B), and the protrusion distance G and retraction distance R invibration, in order that the movement command program for a desiredejector operation can be prepared automatically by referring to theinteractive screen of the manual data input device with display 106. Bydoing so, a device which is more convenient for the user can beprovided.

Also, a movement command program for executing a single or pluraloperation patterns, for example, one or both of the operation patternsshown in FIGS. 3A and 3B may be written in advance in ROM 104 by themanufacturer, and the user is required only to set the data such as thepositions A, B, C, D, and E, the protrusion distance G, and theretraction distance R according to the sizes of the portions of themovable mold 8 and the molded product, so that an ejector operation isperformed by referring to these data when the movement command programis executed.

In the injection molding machine having the servomotor M for ejector andthe control unit 100, the modification or addition of hardware is notneeded at all in applying the present invention, so that the increase inmanufacturing cost due to the addition of new functions is held to aminimum.

We claim:
 1. An ejector control method for an injection molding machine,in which the operation of an ejector mechanism driven by a servomotor iscontrolled according to a movement command program, by which a moldedproduct remaining in a cavity or core of a movable mold fixed to amovable platen is removed from said cavity or core and dropped byprotruding an ejector pin in said cavity or core, said method comprisingthe steps of:making said ejector pin reach a predetermined protrusionlimit position beyond a position where the removal of said moldedproduct from said cavity or core is to be completed, by conductingpositioning confirmation; and making said ejector pin perform aplurality of cycles of reciprocating motion of a short amplitude,without any positioning confirmation, so that said ejector pin neitherretracts beyond a position where the removal of said molded product fromsaid cavity or core is started nor protrudes to said protrusion limitposition.
 2. An ejector control method for an injection molding machineaccording to claim 1, whereinsaid ejector pin is first protruded at apredetermined speed from a predetermined retraction position to saidpredetermined protrusion limit position by one protruding operation; andsaid plural cycles of reciprocating motion of said ejector pin are thenstarted from said protrusion limit position.
 3. An ejector controlmethod for an injection molding machine according to claim 1, whereinthe amplitude of said reciprocating motion is decreased gradually.
 4. Anejector control method for an injection molding machine according toclaim 1, whereinsaid ejector pin is first protruded from a predeterminedretraction position to a protrusion position slightly beyond saidposition where the removal of said molded product from said cavity orcore is started, by a single protruding operation; and said plurality ofcycles of reciprocating motion of said ejector pin are then performedfrom the protrusion position so that a protrusion distance is largerthan a retraction distance of said ejector pin, and said ejector pinreaches said predetermined protrusion limit position by a final advancemotion in said reciprocating motion.
 5. An ejector control method for aninjection molding machine according to claim 1, wherein the positioningconfirmation to confirm that said ejector pin has reached saidpredetermined protrusion limit position is conducted by checking thatthe difference between a position command for giving a command to acontrol unit controlling said servomotor and the actual position of saidservomotor is within a predetermined range.